Its a very simple system an any cryptocurrency are listed in two exchange some time one place price and another price are Change so a people buy a low and they sell these are another exchange that is buying low and sell high system.
Such differences can be seen very rarely and with very few coins this'll happen. In the past when exchanges operated within a specific region or within a country this had been there. Now what we have for trading practice is different. The Exchanges were universal and have the common price. In the past there were people who have been making good money buying it in one country and selling on the other country.